Many tools for forming or shearing purposes comprise a die, i e a cavity. Examples of such tool dies are punching dies, deep drawing dies, powder compaction dies, and cold extrusion dies. Other examples are drawing rings and extrusion dies. Such tools are often subject to strong radial forces, which could easily cause the die to crack. Therefore it is common practice to place the die inside a shrink ring to apply a prestress, a compressive stress, which may counteract the critical tensile stress occuring in the tool during work.
It is precision work to manufacture shrinkage fit dies. Both the core and the surrounding shrink ring must be turned and ground with extremely high precision (.+-.7 .mu.m). Such manufacturing is therefore expensive. Another drawback of this known technique is that the tool-manufacturer must purchase and stock bars of two different types of material, which have to be machined separately. The coursely machined die must then be sent away for heat treatment. Before shrink-fitting, the die must then be ground and adjusted to fit the shrink ring.